In the prior art, a wide range of treatment methods having the purpose of refraction correction in the human eye are known. In this context, the purpose of the operation methods is to alter the cornea selectively so as to influence the light refraction in the eye. A plurality of operation methods are used for this purpose. At present, what is known as laser-assisted in situ keratomileusis, also known as LASIK for short, is the most widespread. In this context, a cornea lamella is initially detached from the cornea surface on one side and folded to the side. This lamella can be detached by means of a mechanical microkeratome, or also by means of what is known as a laser keratome, such as is marketed for example by Intralase Corp., Irvine, USA. After the lamella has been detached and folded to the side, the LASIK operation provides the use of an excimer laser, which removes, by ablation, the corneal tissue which is exposed under the lamella in this manner. After the volume present under the cornea surface has been vaporised in this manner, the cornea lamella is folded back onto the original spot again.
The use of a laser keratome to expose the lamella is advantageous by comparison with a mechanical blade, since the geometric precision is improved and the frequency of clinically significant complications is reduced. In particular, the lamella can be produced with a much more constant thickness if laser radiation is used. The cut edge is also precisely formed, and this reduces the risk of healing difficulties as a result of this boundary surface which remains even after the operation. However, a drawback of this method is that two different treatment devices have to be used, specifically on the one hand the laser keratome for exposing the lamella and on the other hand the laser which vaporises the corneal tissue.
These drawbacks are eliminated in a method which was implemented very recently by Carl Zeiss Meditec. In this lenticule extraction method, a cutting geometry which separates a cornea volume (known as a lenticule) in the cornea is formed in the cornea of the eye by means of a short-pulse laser, preferably a femtosecond laser. This is then removed manually by the operator. One advantage of this method is that the cutting quality is further improved by the use of the femtosecond laser.
Moreover, only one treatment device is now necessary; the excimer laser is no longer used.
A development of the method is referred to in the literature as the SMILE method, in which instead of producing a flap, merely a small opening cut provides access to the lenticule positioned under what is known as the cap. The separated lenticule is removed through this small opening cut, damaging the biomechanical integrity of the anterior cornea less than in LASIK, FLEX or PRK. In addition, fewer nerve fibres in the cornea are cut up in this manner, and this has a demonstrably favourable effect on the restoration of the original sensitivity of the cornea surface. The symptom of dry eyes, which often has to be treated after LASIK, is thus reduced in intensity and duration. Other complications following LASIK, which generally relate to the flap (for example folding, epithelial ingrowth in the flap bed), occur more rarely with no flap.
When producing cutting surfaces in the cornea by means of laser radiation, the optical radiation effect is usually exploited in that an optical aperture is produced by means of individual optical pulses, the duration of which may be between 100 fs and 100 ns. It is also known to introduce individual pulses, the energy of which is below a threshold for an optical aperture, into the tissue or material with an overlap, in such a way that material or tissue separation is achieved in this way too. This idea for producing a cut in the corneal tissue makes a large number of cuts possible.
However, it is common to the different lenticule geometries that a lenticule is produced which—embedded in the treated cornea—corresponds to a conventional concave-convex lens, this lens being able to have much higher orders of correction (cylindrical, spherical aberration etc.). The correction effect in the vision correction carried out by means of lenticule extraction is based on the defined alteration of the radius of curvature of the cornea by removing the lenticule volume. A corresponding lenticule geometry is known to the person skilled in the art from DE102006053120 A1.
For the known method, it is basically irrelevant at what depth the lenticule is removed, so long as a sufficient and predictable effect on the anterior side of the cornea is achieved. It is also irrelevant exactly how the cut shape of the individual cuts is configured, just so long as the thickness of the tissue present between the two cuts follows the known relationships. However, it is thus immediately obvious that a minimum thickness is required for a particular refractive effect which is to extend over an optical zone having a particular extent. This is also known to the person skilled in the art from DE102006053120 A1. The lenticule must not be less than this minimum thickness. In the case of excimer laser correction too, in which tissue is vaporised, there is a corresponding lower thickness of the tissue to be vaporised.
This minimum thickness m of a lenticule is approximated by the following equations.
      m    =                  R        CV            -              R        CV        *            -                                    R            CV            2                    -                      r            2                              +                                    R            CV                          *              2                                -                      r            2                                          R      CV      *        =                  1                              1                          R              CV                                +                                    B              BR                                                      (                                                      n                    C                                    -                  1                                )                            ⁢                                                (                                      1                    -                                          d                      HS                                                        )                                ·                                  B                  BR                                                                        +      F            F    =                  (                  1          -                      1                          n              C                                      )            ⁢              (                              d            C            *                    -                      d            C                          )            
In this context, RCV is the radius of curvature of the anterior side of the cornea, r is the diameter of the optical zone of the lenticule, nc is the refraction power of the cornea, dHS is the vertex distance of the cornea, BBR is the desired correction value (in dpt) and dC is the thickness of the cornea. In this context, the variables F, RCV and dC correspond to the respective state before the correction; the variables corresponding to the state after the correction are marked *. Like the other described relationships, the expression for F is a mathematically precise solution, which may not always be adhered to in reality. The resulting error correction would then have to be compensated in other ways, for example a transformation table for input parameters (nomogram). Finally, the relationship shown here would be obtained again in an approximation suitable for practical use.
For many refractive laser corrections this is unproblematic, but for large correction amounts it is sometimes not possible to carry out the refraction correction in such a way that the entire optical zone of the cornea of the treated eye undergoes the desired correction. The reason for this is the risk involved to the mechanical stability of the treated cornea, which could be deformed in an uncontrolled manner over a long period as a result. As a provisional solution, the correction is sometimes only carried out completely in a central region of the optical zone, and in the surrounding edge zone either no correction or a smaller correction is carried out. In these cases, a higher residual stromal tissue thickness (residual stromal thickness) is obtained, impairing the quality of the optical correction. This quality defect cannot be eliminated using the currently known methods.